Second Round for Nvidia Quad SLI: Technology Matured?. Page 6

You may remember that our early tests of a quad-GPU configuration using Nvidia’s quad SLI technology exposed a number of deficiencies of that idea. We’re going to give quad SLI a second try today to see if it has matured enough to be truly useful.

The parts of the frame are still being combined on a programmable Xilinx XC3S400 array from the Spartan-3 family. It is coupled with two Silicon Image SiI163В TMDS transmitters that can output in dual-link DVI mode at a max resolution of 2560x1600@60Hz. The 3-channel 10-bit ADV7123 DAC working at a frequency of 330MHz allows to connect monitors with an analog interface and use resolutions up to 2048x1536@75Hz. Devices with composite (RCA) or S-Video connectors can’t be attached to the card because the compositing engine and the DAC are located behind the TV encoder integrated into the GPU. To support TV output, the Radeon X1950 CrossFire Edition would require an additional chip to encode TV signal. This would make the Compositing Engine even more complex.

Other than the Compositing Engine, the design of the Radeon X1950 CrossFire Edition is analogous to the Radeon X1950 XTX. Their main parameters like GPU and memory frequencies coincide, too. The card carries eight GDDR4 chips from Samsung (K4U52324QE-BC09) clocked at 1000 (2000) MHz. The GPU works at 650MHz. So, there can’t be a problem of frequency misbalance between the Master and Slave cards that make up a CrossFire tandem.

The Master and Slave are linked with a Y-shaped cable with two DVI connectors and one dual-channel DMS connector. The male-type DVI is attached to the Slave card, and the DMS plugs into the appropriate connector on the Master. The remaining female-type DVI is for connecting the monitor. The CrossFire concept can be illustrated like follows:

As before, the CrossFire system can work in four different modes: Scissor, SuperTiling, Alternate Frame Rendering and Super AA. The Scissor and AFR modes are alike to the SFR and AFR modes in Nvidia’s SLI: the former splits the frame in two parts, the relative size of which is dynamically adjusted by the driver depending on the load on the GPUs, and the latter alternately outputs frames each of which is wholly rendered by one of the GPUs.

The AFR mode ensures better performance scalability in comparison with the Scissor mode because it doesn’t involve any load balancing overhead – each chip processes a whole frame. But it has the same problems as Nvidia’s SLI with render-to-texture operations. This mode has a specific feature: it works only if there is an appropriate game profile in the Catalyst driver. It can be forced for games that do not have such support (set the Catalyst A.I. option at Advanced).

CrossFire’s SuperTiling mode has no analog in Nvidia’s SLI technology. In this mode the frame is tessellated into 32x32 tiles. Each tile is processed by one GPU. This mode works only with Direct3D applications.

The Super AA mode serves the same purpose as SLI AA. That is, it improves the image quality by using advanced full-screen antialiasing techniques. The user is offered 8x, 10x, 12x and 14x Super AA. The number of samples is doubled in 8x and 12x modes: each card in the CrossFire tandem performs 4x or 6x FSAA, respectively, with a shift of the samples. Then the results are combined into the final frame. The 10x and 14x levels of FSAA bring in additional antialiasing, equivalent to 2x super-sampling, to improve the overall antialiasing quality even more. The double number of texture samples in Super AA mode gives you anisotropic filtering equivalent to 32x. Combined with the High-Quality AF option, this ensures an unsurpassed image quality.